I've recently installed kubuntu 9.10 AMD x64, on an HP 6830s. The installation went smoothly, with only one problem: when I try to use the internet, it's incredibly slow. I couldn't install Firefox or anything that requires the internet. I have tried most of the solutions I found through Google, but none of them solves the problem.

Your problem does not seem to be a problem of (K)Ubuntu, but of your router or ISP (see Launchpad for a looong bug report). Therefore, I advise you to not disable IPv6. While this might help in short term, he problem will probably hunt you down again in one or two years when IPv6 takes over from IPv4.

The first thing you should try is updating your router's firmware. This completely solved the problem for me (and others). If this doesn't help, you should bug either your ISP or the router's manufacturer until they fix things.